Effect of monochromatic UV-B radiation on electron transfer reactions of Photosystem II

The adverse effect of low intensity, small band UV-B irradiation (lambda = 305 +/- 5 nm, I = 300 mW m(-2)) on PS II has been studied by comparative me asurements of laser flash-induced changes of the absorption at 325 nm, Delt aA(325)(t), as an indicator of redox changes in Q(A), and of the relative f luorescence quantum yield, F(t)/F-o, in PS II membrane fragments. The prope rties of untreated control were compared with those of samples where the ox ygen evolution rate under illumination with continuous saturating light was inhibited by up to 95%. The following results were obtained: a) the detect able initial amplitude (at a time resolution of 30 mus) of the 325 nm absor ption changes, DeltaA(3)25, remained virtually invariant whereas the relaxa tion kinetics exhibit significant changes, b) the 300 mus kinetics of Delta A(3)25 dominating the relaxation in UV-B treated samples was largely replac ed by a 1.3 ms kinetics after addition of MnCl2, c) the extent of the flash induced rise of the relative fluorescence quantum yield was severely dimin ished in UV-B treated PS II membrane fragments but the relaxation kinetics remain virtually unaffected. Based on these results the water oxidizing com plex (WOC) is inferred to be the primary target of UV-B impairment of PS II while the formation of the 'stable' radical pair P680(+.)Q(A)(-.) is almos t invariant to this UV-B treatment.